Overview
Color image reproduction systems typically include an input device for obtaining a representation of an original image, an output device for generating a replica of the image, and a controlling device that processes signals received from the input device to generate new signals sent to the output device to produce the replica, which preferably is a high-fidelity reproduction of the original image. The controlling device may be implemented by a general-purpose computer with appropriate software and/or hardware for peripheral control and signal processing. Examples of an input device include hand held, flatbed and sheet-fed optical scanners, digital and video cameras, and software applications. In other words, an original image may be sensed or it may be created by a process. Examples of an output device include ink jet, laser and photolithography printers, electrostatic, flatbed and drum plotters, and video displays such as cathode ray tubes, thin-film-transistor and liquid crystal display panels.
Generally, input and output devices use some device dependent color-coordinate system to specify colors. These coordinate systems are often specified in some device-dependent color space that conveniently maps the color coordinates to the color-sensing or color-generating process of the particular device. The term "color space" refers to an N-dimensional space in which each point corresponds to a particular color.
One example of a three-dimensional color space is an RGB space in which point coordinates specify particular amounts of red (R), green (G) and blue (B) colorant that additively combine to represent a specific color. The operation of many scanners and color display devices may be conveniently controlled by signals that are specified in RGB space. An example of a four-dimensional color space is a CMYK color space in which point coordinates specify particular amounts of cyan (C), magenta (M), yellow (Y) and black (K) colorant that subtractively combine to represent a specific color. Another example is the three-dimensional CMY color space. The operation of many ink jet and laser printers may be conveniently controlled by signals that are specified in CMYK space or CMY space. Other color spaces that are related to particular devices are also known.
Many practical devices are capable of sensing or reproducing only a portion of the full range of colors that can be discerned by a human observer. A device "gamut" refers to the range of colors that can be sensed or reproduced by a particular device. For example, the gamut of a particular scanner refers to the range of colors that can be sensed by that scanner and the gamut of a particular printer refers to the range of colors that can be reproduced or printed by that printer.
A scanner gamut is determined by a variety of factors including the spectral response of the optical sensors, the spectral characteristics of color filters, spectral characteristics of the illumination source and the resolution and linearity of analog-to-digital converters.
A printer gamut is determined by a variety of factors including spectral characteristics of colorants such as ink, spectral and porosity characteristics of media such as paper, resolution or dots-per-inch of the printed image, half-toning methods and use of dithering, if any.
A video display gamut is determined by a variety of factors including spectral characteristics of the light emitting material, type of display device, resolution of pixels or video lines, and excitation voltage.
Although it is possible in principle to construct a color image reproduction system by merely connecting an output device directly to an input device, the results generally would not be satisfactory because the device-dependent coordinate systems and color spaces for the input and output devices are generally not the same. Even if the two sets of coordinate systems and color spaces are the same, the fidelity of the reproduced image as compared to an original image would probably be very poor because the gamut of the input device generally is not co-extensive with the gamut of the output device. Values representing "out-of-gamut" colors that are not in the output device gamut cannot be reproduced exactly. Instead, some "in-gamut" color that is in the gamut of the output device must be substituted for each out-of-gamut color.
Color image reproduction systems can achieve high-fidelity reproductions of original images by applying one or more transformations or mapping functions to convert point coordinates in one color space into appropriate point coordinates in another color space. These transformations may be conveniently performed by the controlling device, mentioned above. In particular, with respect to the output device gamut, transformations are used to convert values representing in-gamut and out-of-gamut colors in an input-device-dependent color space (DDCS) into values representing in-gamut colors in an output-DDCS. The mapping of in-gamut colors and out-of-gamut colors is discussed separately.